A switched power source, is a safe idea: when the key is off, you want the socket off, or a charging device could drain your battery if you forgot to pull it. Wiring directly to the battery, with an inline fuse, is fine, but use a relay, connected to a switched circuit, to control it.
I found a 12V accessory outlet, with a spring-closed, rubber sealed door, at Wal-Mart, for around $10. I mounted it to my Voyager's cruise control housing, atop the fake gas tank, wired to a sub-fuse block, switched by a relay. I sealed the wired end with RTV Silicone, to make sure water doesn't get into it; I also sealed the opening the wires feed through, into the false tank compartment, to prevent water getting in.
RTV Silicone comes in black, and clear, and maybe other colors, as well. It is an indispensible tool -- I carry a tube on my bike at all times. I had a lens cover come off of a front signal, on the fairing of my Voyager (screw fell out). I used RTV silicone to secure it on the road, several days away from the end of our bike tour. That was over a year ago -- still holding perfectly. If necessary, RTV silicone is available in every auto parts store, or any hardware store. If you don't have it with you, you can likely pick it up easy enough. Personally, I prefer to carry it with me, though.
A crude, but effective way to waterproof your GPS, would be to 'wrap' it inside a zip-lock baggie, opening down, securing the baggie around the mount's shaft. Not pretty, but effective. Handlebar mounts can be purchased off of e-Bay for typically less than $30, delivered; buy a 'bicycle' mount, big enough to fit your handlebars, and you will be set. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII